In general, materials to form insulating films are exemplified by inorganic materials such as silicon dioxide and silicon nitride; and organic materials such as cyclic olefin copolymers, polyimides, parylenes, amorphous fluorocarbon resins, and polyacrylic resins. Such different materials are used depending on the intended use and prescribed properties.
Conventional liquid crystal displays mainly employed inorganic materials, but the replacement of the inorganic materials with organic materials has started. This is because of recent demands for more flexible displays and for reduced cost.
The inorganic materials form insulating films by vapor deposition technique. In contrast, advantageously, the organic materials can form insulating films (insulating coats) easily and inexpensively by coating technique, allow the resulting insulating films to be lightweight and flexible, and are superior in these points to the inorganic materials. These advantages enable the formation of large-area electronic devices on a mass scale rapidly and inexpensively by coating technique (Non Patent Literature (NPL) 1). As an example of the electronic devices, organic thin film transistors are considered to be producible each by a total of four printing processes including 1) printing of a gate electrode, 2) printing of a gate insulating film, 3) printing of a semiconductor, and 4) printing of source/drain electrodes.
Formulation of the organic materials into inks or pastes is of significance in production of electronic devices by coating technique. Disadvantageously, however, the polyimides have low solubility in solvents. Polyamic acids, which are precursors of the polyimides, have high solubility in solvents, but require heating at high temperatures to be converted into polyimides (Patent Literature (PTL) 1). Disadvantageously, this damages a substrate or a material on which printing is performed.
The parylenes require thermal decomposition by heating so as to form coat layers (PTL 2) and are unsuitable for the production of electronic devices by coating technique.
The amorphous fluorocarbon resins can form coat layers by coating technique. The resulting coat layers, however, have poor wettability and impede the formation of an upper layer thereon by coating technique. This disadvantage has been solved typically by a method of forming an upper layer such as an electrode or an organic semiconductor by vapor deposition technique; a method of performing a coating process of an amorphous fluorocarbon resin after the formation of the upper layer; or a method of forming an adhesion layer of an inorganic material on the coat layer of an amorphous fluorocarbon resin, and then forming an upper layer on the adhesion layer (PTL 3). Disadvantageously, however, these methods require another extra technique in addition to the coating technique to produce electronic devices, or subject to constraints in shapes of the electronic devices.
In contrast, the cyclic olefin copolymers have high wettability with solvents. In addition, the cyclic olefin copolymers each have a low relative permittivity and a high insulation resistance. Disadvantageously, however, the cyclic olefin copolymers have poor solubility in solvents and tend to be precipitated or phase-separated even when formulated into inks. As a possible solution to improve the solubility, a method of chemically modifying cyclic olefin copolymers is known. Disadvantageously, however, the chemical modification causes deterioration of insulation performance (PTL 4 and PTL 5).
Examples of solvents mainly used for the formulation of the organic materials into inks include toluene, xylenes, tetrahydrofuran, dioxane, N,N-dimethylformamide, N-methylpyrrolidone, cyclohexanone, and ethylene glycol ethers. These solvents are, however, excessively highly volatile and thereby impede leveling of the coat layer surface. In addition, the solvents are listed as substances to be controlled as substances of very high concern (SVHC) and are hardly usable industrially. The solvents therefore require substitutes.